Infrared sensitive tube



March 20, 1956 E. E. SHELDON 2,739,244

INFRARED SENSITIVE TUBE Filed May 22, 1951 2 Sheets-Sheet l INVENTOR. fan/M0 [ma/um JMELDflN 197702 Me y March 20, 1956 E. E. SHELDON 2,739,244

NFRARED SENSITIVE TUBE Filed May 22, 1951 2 Sheets-Sheet 2 Z jun/v0 iiuoetsceur 1 47513 4: 6' towmeo [Mom/1. 64am iim'jw limited This invention relates to an improved method and device of intensifying images and refers more particularly to an improved method and device for intensifying images formed by the impingement of infra-red rays on a'fluorescent or other reactive screen, and it is a continuation in part of my U. S. Patent No. 2,555,423, issued June 5, 1951, and has also a common subject matter with applicants U. S. Patent No. 2,603,757, filed on November 5, 1948.

One primary object of the present invention is to provide a method and device to produce intensified infrared images. This intensification will enable to overcome the problems encountered when dealing with weak sources of infra-red.

Another objective of this invention is to make it possible to pick up infra-red images of a long wave length.

Another objective is to provide a method and device to produce shaper images than was possible until now.

Another objective is to amplify or decrease contrast infra-red images.

In order to obtain the objectives of this invention a special infra-red sensitive image tube had to be designed, Fig. 1. This novel infra-red image tube is characterized by elimination of the optical lens system present in other image tubes which resulted in 20-30 fold gain in the light reaching the photocathode. Then by the combined use of a novel electron image amplifier system, of the electronic acceleration and of the electronic image diminution the intensification of the luminosity of the original image exceeding the ratio of 1000-1 was accomplished.

The elimination of the optical system present in other image tubes to focus the fluorescent image on the photocathode of the tube was accomplished by positioning within the infra-red sensitive image tube of the screen, consisting of combination of an infra-red transparent, light reflecting layer 2, of infra-red sensitive phosphor 3, and of the photoemissive layer 5. All layers are placed in close apposition to each other to prevent the loss of definition. The fluorescent and photoemissive layers are separated only by a very thin light transparent, chemically inactive, barrier layer 4 of a thickness not exceeding 0.15 millimeter. The previous combinations of fluorescent and photoemissive layers were not successful because of detrimental chemical interaction of both layers, due to lack of a barrier between them, therefore the introduction of light transparent barrier layer represents a very important part of this invention. The photoemissive layer 5 is of a semi-transparent type. This layer is characterized by emission of electrons on the side opposite to the side of the incident light. The photoelectrons emitted from the photoemissive layer in a pattern corresponding to the incident light pattern are fo'cussed by means of magnetic and/or electric fields on the novel image amplifying system 7.

The amplification section of the tube '7 consists of one or a few screens each of them composed of a very thin light-reflecting, electron pervious layer 8, of a fluorescent layer 9 and of a photoemissive layer 11 in close appositates Patent ice 2 tion to each other. It is necessary to include a very thin light transparent, chemically inactive barrier layer 10 between the fluorescent and photoemissive layers in order to prevent their chemical interaction, which should be of a thickness not exceeding 0.15 millimeter. The electrons from the pick-up section of the image tube are focussed by magnetic or electrostatic fields on the fluorescent layer of a screen described above. The luminescence of the fluorescent layer of the amplification screen will cause the emission of electrons from the photoemissive layer of the screen. This process can be repeated a few times, using a few screens described above resulting in 10-100 times intensification of the original electron image.

The electrons leaving the amplifying section are accelerated by means of high voltage electro-static fields. The accelerating system can be of a conventional type well known in the-art. Much better results with higher voltages willbe achieved with an electro-static multi-lens system 19a.

Next the electron image is demagnified which results in its additional intensification. The electron diminution of the image, in order to gain its intensification is well known in the art, therefore does not have to be described in detail.

The diminished electron image is projected on the fluorescent screen at the end of the tube 21 where it can be viewed by the observer directly or by means of an optical magnifying eye piece 23, through the light transparent end wall of the tube 22.

The use of an optical eye piece to magnify optically the electronically diminished image appearing on the fluorescent screen, is also well known in the art, therefore does not need further description.

The combination of the above described features of the infra-red sensitive image tube allows to obtain intensification of the original infra-red image which was the primary objective of this invention. Having such a marked intensification of the original infrared image it will be possible now to use a much finer grain of fluorescent screens 3 and 21 than was practical until now and to improve this way detail and contrast of the final image, which was another purpose of this invention.

The invention will appear more clearly from the following detailed description when taken in connection with the accompanying drawings by way of example only, preferred embodiments of the inventive idea.

Fig. 1 is a cross-sectional view of the infra-red sensitive image tube;

Fig. 2 is a cross-sectional view of the curved infra-red sensitive image tube;

Fig. 3 is a cross-sectional view of the optical system for the infra-red sensitive curved image tube;

Fig. 4 is a cross-sectional view of the modification of the infra-red sensitive image tube;

Fig. 5 is a cross-sectional view of a modification of the composite infra-red sensitive image tube; and

Fig. 6 is a cross-sectional view of a modification of the infra-red sensitive image tube adapted for flickering images.

Figs. 7, 8 and 10 are cross-sectional views of infra-red tubes adopted for intensification and storage.

Fig. 9 is a plan view of the storage target.

The infra-red image 32 is projected on the face 1 of the image tube shown in Fig. 1. The face 1 of the image tube must be of a material transparent to the type of radiation to the used. Inside of the face of the tube there is a very thin visible light reflecting layer 2, but

transparent to infra-red, ultra-violet or gamma rays, such as of gold, which prevents the loss of light from the adjacent fluorescent layer 3. An extremely thin barrier layer 4 separates the fluorescent screen 3 from the adjacent photoemissive layer 5. The fluorescent 3 and photoemissive layers 5 should be correlated so that under the influence of the particular radiation used there is obtained a maximum output of photoemission. More particularly the fluorescent layer should be composed of a material having its greatest sensitivity to the type of radiation to be used, and the photoemissive material likewise should have its maximum sensitivity to the wave length emitted by the fluorescent layer. In some cases the refleeting layer 2 may be preferably omitted.

The selection of a fluorescent material responsive to infra-red radiation depends on the purposes to be obtained. Alkaline earth sulphides or selenides activated by cerium and sarnarium or by europium and samarium should be used if the maximum sensitivity to infra-red is desired. The best of them is strontium sulphide activated by cerium and Samarium. The next in sensitivity is strontium sulphide activated by europium and samarium or lanthanum sulphides with activators. If minimum background is desired CaS phosphor is more advantageous. If sensitivity to long waves of infra-red is required CaS, SrS or ZnSCdzCu may be indicated. By proper selection of activators in alkaline earth sulphides and selenides we can get visible fluorescence in the desired range of spectrum. We can also increase or decrease the contrast of a fluorescent image produced by infra-red radiation by a proper use of activators. In order to decrease the contrast, an activator should be selected which makes phosphor relatively less sensitive to strong doses of exciting radiation than to small doses of the same. In order to increase the contrast the activator to be used must make the phosphor relatively more sensitive to strong doses of the exciting radiation than to small doses of said radiation. Some infra-red fluorescent layers work best when kept at a low temperature such as K=90.

The infra-red sensitive phosphors described above require excitation before they become sensitive to infra-red. They can be excited by irradiation with ultra-violet, X-rays or cathode rays. Following such an excitation they respond to infra-red images by producing a fluorescent image which may be in the visible or in the invisible range of spectrum, depending on activators. In the operation of my device the fluorescent layer 3 of the composite photocathode is excited, before it is used, by ultra-violet light or by a gamma-ray beam. Next it is exposed to the examined infra-red images. The infra-red rays cause fluorescence of the previously excited phosphor 3, which has the pattern of the infra-red image. The fluorescent light image from the layer 3 passes through the light transparent layer 4 and acts on the photoemissive layer 5 causing emission of electrons therefrom having the pattern of said fluorescent light image.

The satisfactory photoemissive materials for the composite photocathode will be caesium oxide, caesium oxide activated by silver, caesium with antimony, caesium with bismuth or arsenic or antimony, with lithium or potassium. The barrier layer 4 between the fluorescent and photoemissive surfaces can be an exceedingly thin transparent film of mica, glass, ZnFz, of organic substance such as nitrocellulose or gelatine, of silicon or of a suitable plastic, or of a conducting material such as known in the trade under the name Nesa and which is a tin-oxide.

The thickness of the separating layer should not exceed 0.15 millimeter and preferably should be of a few microns only, as explained in applicants Patent No. 2,603,757. The light transparent separating layer 4 in the photocathode 5a may be deposited on the fluorescent layer 3 so that it doesnt require any support by the walls of the tube. In modification of the composite photocathode the light transparent separating layer 4 may be attached to the walls of the tube by means of metallic rings and may provide the support for other layers.

The reflecting layer 2, the fluorescent layer 3, the sepa-,

The electron image obtained in the pick-up section is now transferred to the first screen of the amplifying section 7 by means of focussing magnetic or electrostatic fields which are indicated, since they are well known in the art and would only serve to complicate the illustration.

The amplifying section 7 uses one or a few successively arranged special screens, each of them consisting of an electron pervious, light-reflecting layer 8, of a fluorescent layer 9, of a light transparent barrier layer 10 and of a photoemissive layer 11. Fluorescent substances that may be used are willemite or other zinc silicates, zinc selenides, zinc sulphides, calcium fluoride or calcium tungstate with or without activators. The satisfactory photoemissive materials will be caesium oxide, caesium oxide activated by silver, caesium with antimony, with bismuth or arsenic or antimony with lithium or potassium. The barrier layer 10 between the fluorescent and photoemissive surfaces can be an exceedingly thin transparent film of mica, glass, ZnFz, or organic substance such as e. g. nitrocellulose or gelatine, of silicon or of a suitable plastic or of conducting material known as Nesa. The separating layer 10 should be as thin as possible and should not exceed the thickness of 0.15 millimeter. The amplification achieved by this system results in marked intensification of the original image.

In some applications it may be preferable to use in conjunction with amplifying system the electron multiplier section 6 consisting of one or a few stages of secondary electron multipliers which serves to intensify further the electronic image. In such a case the electron image from the pick-up section of the tube is focussed by means of magnetic field on the first stage of the multiplier section. The secondary electrons from the first stage are focussed the same way on the second stage of the multiplier section and so on. CsO:Cs or Ag:Mg multipliers provide a good secondary electron emission.

The electrons emerging from the amplifying section are now accelerated and imaged by means of electromagnetic or electro-static fields 1% to the desired velocity, giving thus further intensification of the electron image. Next the electron image is diminished by means of electromagnetic or electro-static lenses 1% to the desired size, resulting in image intensification proportional to the square power of the linear diminution and is projected through the electron pervious, light-reflecting aluminum layer 20 on the fluorescent screen 21 made of fine grains of ZnO, Zn silicates or ZnS with appropriate activators where it can be viewed by the observer. in some cases it may be more desirable to have the fluorescent screen mounted outside of the vacuum tube, in such cases thin electron transparent layer of chromium or aluminum is placed on the end wall 22 of the vacuum tube made of fernico glass. The image appearing on the fluorescent screen can be viewed directly or by means of an optical eye-piece 23 giving the desired optical magnification of the image. In other cases the fluorescent screen 21 is substituted by photographic layer. or by photographic layer in combination with fluorescent screen permitting thus to obtain a permanent record of electron image. Also electrosensitive paper such as Teledeltos or selenium coated metal plate sensitized by an electrical charge may be used for recording.

In another alternative of this invention the tube is curved (Fig. 2) and the electron beamv isdeflectedby proper magnetic fields. This arrangement will prevent the positive ions from reaching the photoemissive section.

A special optical system using a few mirrors (Fig.3) or a periscopic arrangement of lenses and mirrors 33 is provided to enable the examiner to correlate accurately the intensified image 35 producedby the curved tube with the investigated object 32.

i A modification of the infra-red sensitive image tube is shown in Fig. 4. The infra-red image 32 is projected onto photocathode 37 which consists of a photoemissive mate'- rial. Good photoemissive materials are CsO Ag, caesium or lithium on bismuth or arsenic. The infra-red image is converted in said photocathode into a photoelectron beam having the pattern of said infra-red image. The photoelectron beam is accelerated or focussed on the image amplifying screen 7 in the same way as was described above and illustrated in the Fig. l. The rest of the operation of this image tube is the same as described above.

In some instances it is preferable to use the quenching effect of the infra-red radiation on the fluorescence or phosphorescence of a fluorescent material. The quenching effect means suppression of fluorescence by means of infra-red radiation. This arrangement is shown in Fig. 5. The phosphor 3c is excited by a beam of ultraviolet or gamma rays. Then the infra-red image 32a is projected on the phosphor. Infra-red rays suppressing fluorescence produce a negative image on the phosphor corresponding to th original infra-red image. The best phosphors for this application are sulphides, especially ZnS activated by copper and cobalt or by copper and cadmium or iron. The fluorescence of the fluorescent layer 30 having imprinted on it the negative infra-red image acts on the photoemissive layer 5 producing an electron emission therefrom which also will have the pattern of said infra-red image. The rest of the operation of this image tube is the same as described above. The best quenching results are obtained when phosphor is kept at a low temperature such as K 90.

in some instances it is important to remove the flicker from the examined infra-red image. In such a case the fluorescent layer 3 or 30 has to be composed of a few layers or" phosphors. This arrangement is shown in Fig. 6. The first fluorescent layer 3:: facing the infra-red image will be of a phosphor sensitive to infra-red images as described above. The second fluorescent layer 3b will be a phosphor which can be excited with the fluorescence of the first fluorescent layer 3 and which when excited will have a required persistence of fluorescence or of phosphorescence. Good phosphors for this purpose are CdScCu; or ZnCdS. In some cases a conducting layer 2, sucn as of gold, which was described above and which is reflecting to the visible light but transparent to infrared or ultra-violet rays, should be mounted adjacent the layer 3a.

in some cases it may be useful to add a third fluorescent layer. The fluorescent layers in question may be adja cent to each other or may be separated by an extremely thin separating layer which should be chemically inactive and transparent to the fluorescent light emitted by the fluorescent layer 35:. The separating layer 4a should also have reflecting properties for the fluorescence of the next adjacent fluorescent layer 35 in order to exploit fully its emission. It should be understood that the above described construction can be applied in all modifications of the screens described in specification in which a fluorescent layer is used. The separating layer 4a should be as thin as possible and should not exceed the thickness of 0.15 millimeter.

Further improvement of this infra-red intensifying tube was obtained by including a storage system in said tube. The use of an infra-red storage tube 40 allows to overcome completely the flicker and what is more important it improves markedly signal to noise ratio resulting in pictures of much better detail and contrast.

In this modification of my invention, shown in Fig. 7, the invisible infra-red image of the examinedobject is converted by the composite photocathode 5a which has been described above into a photoelectron image. The photoelectron image is accelerated by the electrode 47 and is focused by the magnetic or electrostatic fields 48 on the perforated storage target 41. The storage target is shown in Fig. 9 and consists of a thin perforated sheet of woven conducting wire mesh 41a. On the side of the target opposite to the photocathode there is deposited by evaporation storage material 41b, such as CaFz, in such a manner that openings 41c in the target should not be occluded. In some cases on the side of the target facing the photocathode there is deposited by evaporation a thin metal coating.

Between the photocathode and the storage target, in a close spacing to the target, there is mounted a fine mesh conducting screen 42. On the side of the storage target, opposite to the photocathode there is disposed a mesh metal electrode 43, which repels electrons during the writing phase of operation and attracts electrons during the reading phase. Adjacent to said meshed metallic screen there is disposed a fluorescent screen 44 provided with a metallic light reflecting electron transparent layer 44a such as of aluminum. The reflector electrode 43 during writing is kept at the potential negative to the photocathode 5a. Therefore the photoelectrons transmitted through the perforated target are repelled by said reflector electrode and have to fall back on the storage target 41 and deposit thereon varying charges at successive points according to the pattern of infra-red image. The best way of operating my system is to have the storage target surface at zero potential or at photocathode potential and then to write on it positive, it means to deposit positive charges. This can be accomplished by adjusting the potential of the surface of the storage target so that its secondary emission is greater than unity.

The photoelectrons having the pattern of infra-red image after passage through openings in the target are repulsed back by the reflector electrode 43 because in this phase of operation its potential is lower than that of the storage target. The impingement of photoelectron beam causes secondary electron emission from the target 41 greater than unity. The secondary electrons are drawn away by the mesh screen 41a of the storage target which is connected to the source of positive potential. As a result a positive charge image is formed on the perforated target 41 having the pattern of the original infra-red image. This charge image can be stored in the target as long as 50 hours, if the storage material is CaFz.

In the reading phase of operation of my system the target 44 is scanned by slow electron beam 50 from the electron gun 52. The electron beam is focused by magnetic or electrostatic field 49 and is decelerated by the electrode 42 which may be in the form of a ring or of meshed screen. The deflecting fields and synchronizing circuit are not shown in order not to complicate the.

drawings. It is obvious that all fields controlling the scanning beam are'inoperative during the writing phase of the operation. In the same way the fields controlling the photoelectron beam are not operating during the reading phase of the operation. A part of the scanning electron beam passes through the perforations 410 in the target 41. The charge image on the target controls the passage of the scanning electron beam 50 acting in the similar manner to a grid in the electron tube.

The electron beam 50 in the reading phase of operation passes through the openings in target 41, is modulated by the stored charges on it and strikes the fluorescent screen 44 producing thereby a light image having the pattern of the original infra-red image. The fluorescent screen 44 is provided with an electron transparent, light reflecting backing 44a such as of aluminum.

At the same time the open mesh metallic screen 43 may be used as a collector for electrons to be converted into video signals and transmitted to distant receivers.

In case no transmission of storage image is desired the image reproduced on the fluorescent screen 44 can be markedly intensified by using instead of the scanning electron beam 50, a broad electron beam from electron gun 52 covering all of the storage target 41, or a flat ribbon scanning beam covering one line of the image.

Video signals can be obtained not only from the transmitted electrons of the scanning beam but as well from the electrons 50a of said scanning beam returning to the electron gun. This part of the electron beam is also modulated by the charge image on the target 41 but is of reverse polarity than the transmitted electrons. The nontransmitted part of the scanning electron beam returns to the multiplier section 46. They are multiplied there and then are converted into video signals. This arrangement by using multiplication of electrons allows a marked.

intensification of video signals.

The returning electron beam 50a contains two groups of electrons. One group are electrons which were refiected specularly from the target. Another one is electrons which were reflected non-specularly, which means were scattered. These two groups can be separated from each other before reaching the multiplier. There are many ways to separate these two groups of electrons, all well known in the art. The best method is to introduce an additional helical motion into a primary scanning beam. Then the scattered electrons in the returning beam will be on one side of the specularly reflected electrons. Therefore it will be possible to direct scattered electrons into aperture of the multiplier, While stopping the reflected electrons by the edge of the multiplier aperture. sensitivity of the system because it reduces the inherent shot noise of the scanning electron beam.

Video signals have the pattern of the original infra-red image. They are amplified and transmitted by coaxial cable or by high frequency waves to receivers. Receivers may be of various types such as kinescopes, facsimile receivers, in combination with electrographic cameras and others may be used to reproduce images for inspection or recording.

The focusing, accelerating and deflecting fields, as well as synchronizing circuits are not shown as they are well known in the art and would only complicate the drawings.

After the stored image has been read and no further storage is desired it may be erased by the use of the scanning electron beam by adjusting the potential of the storage target to the value at which the secondary electron emission of its storing surface is below unity. In such a case the target will charge negatively to the potential of the electron gun cathode. The potential of the reflector in the erasing phase of operation must be more negative than that of the storage target, so that the scanning electron beam will be repelled to the target.

It is obvious that the composite photocathode, the electron gun and the'perforated target may be disposed in many different ways and it is to be understood that the various modifications of their mutual arrangement come within the scope and spirit of my invention. One of such modifications is shown by the way of example only in Fig. 8.

The infra-red tube 53 operates in the same way as the tube 70, the only difierence being the photoelectron image is projected on the storage target at an angle which requires the use of arcuate focusing fields 48a.

Between the photo-cathode 5a and the storage target 64, in a close spacing to the target, which was described above, there is mounted a fine mesh conducting screen 65. On the opposite side of the storage target thereis disposed a meshed electrode 71 which acts as an electron mirror during the writing phase of operation. The

reflector electrode '71 during writing is kept at the poten-' tial negative in relation to the photocathode 5a. There The use of scattered electrons increases markedly Q fore the photoelectrons from the photocathode which passed through the perforations in the target are repulsed by it and have to fall back on the storage target '64 and deposit thereon varying charges at successive points. The best way of operating my system is to have the storage surface at zero potential or at photocathode potential and then to write on it .positive, which-means to deposit positive charges. This can be accomplished by adjusting the potential of the surface of the storage target so that its secondary emission is greater than unity.

The photoelectron image is accelerated by a ring electrode :73 and is focused by magnetic or electrostatic fields 66. The photoelectron image after passage through openings in the target strikes the reflector electrode 71 and is repulsed back because in this phase of operation its potential is lower than that of storage target. The impingement of 'photoelectron beam causes secondary electron emission from the surface 64b of the target 64 greater than unity. The secondary electrons are drawn away by the mesh screen 64a of the storage target which is connected to the source of positive potential. As a result a positive charge image is formed on the perforated target 64 having the pattern of the original infra-red image. This charge image can be stored in the target as long as 50 hours. The photoelectron image may be also stored in the form of .a charge image on the side of the storage target facing the photocathode. In such a case the storage surface 64b of the target should face the photocathode and the mesh. electrode '71 is activated during the writing phase, for collecting the secondary electrons.

In the reading phase of operation of my system the target 64 is scanned by a slow electron beam 62 from the electron gun '72. The electron beam is focused by magnetic or electrostatic fields 67 and is decelerated by the mesh electrode 71 or by a ring electrode 74. The deflecting fields and synchronizing circuits are not shown in order not to complicate the drawings.

It is obvious that all fields controlling the scanning beam are inoperative during the writing phase of the operation. In the same way the fields controlling the photo electron beam are not operating during the reading phase of the operation. The scanning electron beam 62 is modulated by the stored charge image. transmitted through the openings in the storage target 64, is collected by the mesh collector 65, and is converted into video signals in the usual manner. Another part 62a of the scanning electron beam is returning to the electron gun '72, isdiverted to the multiplier 73 and after multiplication therein is converted into, video signals. Video signals having the pattern of the original infra-red image will be amplified and transmitted by coaxial cable or byhigh frequency waves to the receivers. Receivers maybe of various types such as kinescopes, facsimile receivers, in combination with electrographic cameras, and others may be used to reproduce images for inspec tion or for recording.

In another modification of my invention shown in Fig. t0, the infra-red sensitive pick-up tube 76 has a composite storage target 77. The infra-red image is converted in the photocathode 77 into photoelectron beam having the pattern of the infra-red image. The photoelectron image is accelerated and focused on the perforated storage .target 79. The target consists of a thin perforated light transparent dielectric layer such as glass 83 or of oxidized aluminum. Also a metallic mesh screen can be used instead of a perforated glass. In such a case however a light transparent dielectric layer 82 must be deposited on the mesh screen and in such a manner that openings in the screen remain unobstructed. On the side of the dielectric layer facing the pbotocathode is deposited a fluorescent layer 81 also in such a manner that the openings in the taregt are unobstructed. On the side of the fluorescent layer facing the photocathode is deposited a light reflecting layer such as of aluminum. On the side of the dielectric layer 83 which is away from the photo cathode is deposited a photoemissive layer 84 in such a manner that openings in the target are not obstructed. Photoemissive layer 84 may be continuous or in the form of a mosaic.

The photoelectron beam causes fluorescence of the layer 81. The fluorescent light passes through glass layer 3 83 and causes emission of electrons from the photoemissive layer 84. The emitted photoelectrons are led away A part of it 62b is i with by adjacent collecting mesh screen 85. As a result a positive charge image is stored on the layer 84. This stored charge controls the passage of the scanning electron beam 87 in the same manner as was described above. The transmitted electrons of the scanning beam will strike the fluorescent screen 89 having a light reflecting electron transparent backing $3 and will reproduce therein the infra-red image. The transmitted electrons are focused on the fluorescent screen 89 by means of magnetic or electrostatic fields which are well known in the art.

Storage of infra-red images may also be accomplished by using a light feed-back system. The fluorescent infrared image in this modification of my invention is reproduced on the face of the image tube as was explained above, is projected on a television pick-up tube and produces a photoelectron image therein. The photoelectron image is converted by the pick-up tube into video signals in the manner well known in television. For the purpose of this invention any type of television pick-up tube such as photoernissive type, photoconductive type, or of photovoltaic type may be used. Video signals are sent from the pick-up tube to the kinescope again and reproduce there the fluorescent image. The fluorescent image is again projected on the pick-up tube to produce again photoelectron image. In this way an endless stream of fluorescent light images is produced so that the fluorescent image may be inspected for a desired time Without maintaining the infra-red exposure.

My device can be also used for storing images before their intensification. Images formed by any exciting radiation such as ultra-violet, gamma, X-rays and cathode rays will be stored in the fluorescent layer 3 or 3a for a long time and will be released therefrom as a fluorescent image when said fluorescent layer is stimulated by infra-red radiation. Conversely the infra-red images may be stored for a limited time by using the quenching effect shown in Fig. 5. The infra-red image is projected on the fluorescent layer 3c after it has been excited by the action of ultraviolet or cathode rays and is stored therein as a negative fluorescent image.

The storage tube 76 described above may be also used for the transmitting of images. In such a case electrons of the scanning beam 87 which are transmitted through the target 79 are made topass through an apertured electrode disposed between the storage target 79 and the end wall of the tube. By applying suitable deflection fields to the transmitted electrons they are made to pass through the aperture in the electrode in succession corresponding to various image points. The electrons which pass through the aperture are fed into a multi-stage multiplier for intensification. The electrons emerging from the multiplier are converted over a suitable resistor into video signals in a manner well known in television. Video signals are sent to receivers by coaxial cable or by high frequency waves to reproduce a visible image. In the appended claims it is to be understood that the term a major portion of the surface of the screen includes construction in which the above mentioned layers cover substantially the entire surface of said screen. Phosphors are materials which emit luminescent light when excited by radiation in contra-distinction to scotophores, such as alkali halides. These differences are well known in the art, as evidenced by the standard text-books such as Luminescence of Solids by H. Leverenz and published by J. Wiley & Sons. Furthermore, the term phosphors indicates broadly luminescent materials emitting luminescent light regardless of the duration of said. luminescent light emission, and apply to luminescent materials with or without binders for the same. The term luminescent means having an even surface indicates that luminescent layer or layers have a surface free from artificially produced grooves, protuberances, voids or the like.

It will thus be seen that there is provided a device in which the several objects of this invention are achieved and which is well adapted to meet the conditions of pr'ac tical use.

As various possible embodiments might be made of the above invention, and as various changes might be made in the embodiment above set forth, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

Having thus described my invention I claim as new and desire to secure by Letters Patent:

1. A vacuum tube comprising a composite screen having a continuous, pervious to electrons, electrically conducting layer extending continuously over a major portion of one surface of said screen, luminescent means comprising a plurality of different from each other phosphors emitting luminescent light and a light transparent, continuous and electrically conducting layer extending continuously over a major portion of the opposite surface of said luminescent means.

2. A vacuum tube comprising a composite screen having a continuous, pervious to electrons and electrically conducting layer extending continuously over a major portion of one surface of said screen, luminescent means comprising a plurality of different from each other phosphors emitting luminescent light and in contact with each other, and a light transparent, continuous and electrically conducting layer extending continuously over a major portion of the opposite surface of said luminescent means, said screen being in contact with the end wall of said tube.

3. A vacuum tube comprising a composite screen having a continuous, pervious to electrons, electrically conducting and visible light reflecting layer extending continuously over a major portion of one surface of said screen, luminescent means comprising a plurality of different from each other phosphors emitting luminescent light and in contact with each other, and a light transparent, continuous and electrically conducting layer extending continuously over a major portion of the opposite surface of said luminescent means, said screen being in contact with the end wall of said tube.

4. A vacuum tube comprising within said tube a composite screen having a continuous, pervious to electrons and electrically conducting layer extending continuously over a major portion of one surface of said screen, luminescent means comprising a plurality of different from each other phosphors emitting luminescent light and in contact with each other, and a light transparent, continuous and electrically conducting layer extending continuously over a major portion of the opposite surface of said luminescent means, said screen being in contact With the end wall of said tube and having a planar shape.

5. A vacuum tube comprising a composite screen having a continuous, visible light reflecting, pervious to electrons and electrically conducting layer extending continuously over a major portion of one surface of said screen, luminescent means comprising a plurality of different from each other phosphors emitting luminescent light and in contact with each other, and a light transparent, continuous and electrically conducting layer connected to a source of direct electrical current and extending continuously over a major portion of the opposite surface of said luminescent means, saidscreen being in contact with the end wall of said tube and having a curved shape.

6. A vacuum tube comprising within said tube a composite screen having a continuous, pervious to electrons and electrically conducting layer extending continuously over a major portion of one surface of said screen, luminescent means comprising a plurality of different from each other phosphors emitting luminescent light and in contact with each other, and a light transparent, continuous and electrically conducting layer extending continuously over a major portion of the opposite surface of said luminescent means, said screen being in contact with the end wall of said tube and having a convex shape.

7. A vacuum tube comprising within said tube a com" posite screen having a continuous, pervious to electrons, electrically conducting and visible light reflecting layer extending continuously over a major portion of one surface of said screen, luminescent means comprising a pluraiity of different from each other phosphors comprising a compound of the group consisting of sulphides and selenides and emitting luminescent light and in contact with each other, and a light transparent, continuous and electrically conducting layer connected to a source of direct electrical current and extending continuously over a major portion of the opposite surface of said luminescent means, said screen being in contact with the end wall of said tube.

8. A device as defined in claim 1, which comprises in addition photoelectric means.

9, A vacuum tube comprising a composite screen having a continuous, pervious to electrons and electrically conducting layer, a plurality of different from each other phosphors emitting luminescent light, luminescent light emitted by one of said phosphors producing emission of the luminescent light from another one of said phosphors, and a light transparent, continuous and electrically conducting layer.

lO. A vacuum tube comprising a composite screen having a continuous, pervious to electrons, electrically conducting and visible light reflecting layer extending continuously over a major portion of one surface of said screen, luminescent means comprising a plurality of different from each other phosphors emitting luminescent light and in contact with each other, a light transparent, continuous and electrically conducting layer extending continuously over a major portion of the opposite surface of said luminescent means, said screen having a convex shape.

11. A vacuum tube comprising within said tube a composite screen having a continuous, pervious to electrons, electrically conducting and visible light reflecting layer extending continuously over a major portion of one surface of said screen, luminescent means comprising a plurality of different from each other phosphors emitting luminescent light, comprising a compound of the group consisting of sulphides and selenides, and in contact with each other, a light transparent, continuous and electrically conducting layer extending continuously over a major portion of the opposite surface of said luminescent means,

screen furthermore being in contact with the end wall of said tube.

15. A vacuum tube comprising a composite screen having a continuous and electrically conducting layer extending continuously over a major portion of one surface of said screen, luminescent means comprising a plurality of separate layers of different from each other phosphors emitting luminescent light, said phosphor layers being disposed in separate and parallel to each other planes, at light transparent, continuous and electrically conducting layer extending continuously over a major portion of the opposite surface of said luminescent means, and photoelectric means.

16. A vacuum tube comprising a composite screen hav ing a continuous and electrically conducting layer, luminescent means comprising a plurality of difierent from each other phosphors emitting luminescent light, luminescent light emitted by one of .said phosphors producing emission of the luminescent light from another one of said phosphors, and a light transparent, continuous and electrically conducting layer extending continuously over a major portion of the surface of said luminescent means and adjacent to said luminescent means. j

17. A composite screen comprising a continuous and electrically conducting layer, luminescent means comprising a plurality of separate layers of different from each other phosphors emitting luminescent light, luminescent light emitted by one of said phosphors producing emission of the luminescent light from another one of said phosphors, said phosphor layers being disposed in successive and separate planes, and a light transparent, continuous and electrically conducting layer.

18. A composite device comprising a continuous and electrically conducting layer, luminescent means having on one side thereof said continuous and conducting layer and comprising a plurality of separate layers of different from each other phosphors emitting a fluorescent light, major surfaces of said phosphor layers being disposed in separate and facing towards each other planes, a light transparent, continuous and electrically conducting layer adjacent to said luminescent means and extending continuously over a major portion of the opposite surface of said luminescent means, and photosensitive means.

said screen having a convex shape and being in contact with the end wall of said tube.

12. A vacuum tube comprising within said tube a composite screen having a continuous, pervious to electrons and electrically conducting layer extending continuously over a major portion of one surface of said screen, luminescent means having an even surface and comprising a plurality of different from each other phosphors emitting luminescent light, said phosphors being in contact with each other, a light transparent, continuous and electrically conducting layer adjacent to said even surface of said luminescent means, said light transparent conducting layer being connected to a source of direct electrical current and extending continuously over a major portion of the opposite surface of said luminescent means, said screen furthermore having a convex shape.

13. A device as defined in claim 9, which comprises in addition photoelectric means.

14. A vacuum tube comprising a composite screen having a continuous, pervious to electrons and electrically conducting layer extending continuously over a major portion of one surface of said screen, luminescent means having an even surface and comprising a plurality of dhferent from each other phosphors emitting luminescent.

light and a light transparent, continuous. and electrically conducting layer compiising a metallic oxide extending continuously over a major portion of the opposite surface of said luminescent means and in contact with said even surface of said luminescent means, said composite 19. A composite device comprising a continuous and electrically conducting layer, luminescent means comprising a plurality of different from each other phosphors, luminescent light emitted by one of said phosphors producing emission of the luminescent light from another one of said phosphors, and a light transparent, continuous and electrically conducting layer extending continuously over a major portion of the surface of said luminescent means.

20. A composite device comprising a continuous and electrically conducting layer, luminescent means having said continuous and conducting layer on one side thereof and comprising a plurality of different from each other phosphors, luminescent light emitted byone of said phosphors producing emission of the luminescent light from another one of said phosphors, major surfaces of said phosphor layers being disposed in parallel to each other planes, a light transparent, continuous and electrically conducting layer mounted on the opposite side of said luminescent means, and'photosensitive means.

21. A composite screen comprising a continuous and electrically conducting layer, luminescent means comprising a plurality of separate'layers of different from each other phosphors emitting luminescent light, major surfaces of said phosphor layers being disposed infacing towards each other planes, 2. light transparent, continuous and electrically conducting layer extending continuously over a majorportion of the surface of said luminescent means, and photoelectric means.

.(Refereuces on following page) References Cited in the file of this patent UNITED STATES PATENTS Busse Oct. 24, 1939 Morton Feb. 6, 1940 Victoreen Feb. 13, 1940 Lubszynski et al. Oct. 7, 1941 Hergenrother Apr. 21, 1942 14 Kallmann et al. Mar. 14, 1944 Cage Feb. 19, 1946 Skellett Dec. 26, 1950 Sheldon June 5, 195]. Sheldon June 5, 1951 Marshall Sept. 30, 1952 

19. A COMPOSITE DEVICE COMPRISING A CONTINUOUS AND ELECTRICALLY CONDUCTING LAYER, LUMINESCENT MEANS COMPRISING A PLURALITY OF DIFFERENT FROM EACH OTHER PHOSPHORS, LUMINESCENT LIGHT EMITTED BY ONE OF SAID PHOSPHORS PRODUCING EMISSION OF THE LUMINESCENT LIGHT FROM ANOTHER ONE OF SAID PHOSPHORS, AND A LIGHT TRANSPARENT, CONTINUOUS AND ELECTRICALLY CONDUCTING LAYER EXTENDING CONTINUOUSLY OVER A MAJOR PORTION OF THE SURFACE OF SAID LUMINESCENT MEANS. 